Abstract

Although it is well accepted that the ultrafast manipulation of spins or magnetization in solid promises potential applications in coherent terahertz (THz) radiation source, spintronics and quantum information processing, their performance is significantly limited by the weak coupling between radiation field and magnetic dipole oscillation. For such ‘weak’ magnetic system, we propose an effective and simple route based on the cavity-based phase modulation technique towards the efficient energy extraction, demonstrated via controlling the magnetic dipole THz radiation generated in the nonlinear Raman process from antiferromagnetic (AFM) NiO. An asymmetric coupled Fabry-Pérot (FP) cavity is constituted by simply placing a metallic planar mirror in the vicinity of a NiO slab. The energy-extraction (THz radiation) can be effectively manipulated by changing the NiO-mirror distance to modulate the phase relation between the magnetic wave and the induced magnetization in NiO. The distinct radiation control can be observed and the experiments are well explained by numerically analyzing the radiation dynamics that highlights the role of phase modulation during the radiation process.

Figures (4)

(a) Schematic of the setup for detecting the THz radiation. (b) Schematic of the asymmetric Fabry-Pérot cavity with the tunable distance between NiO and silver mirror; z=z0 is the NiO/air boundary where THz waves emit into free space. (c) Measured time-domain waveform of the radiated THz electric field at different distances of the mirror, forward-radiation of the bare NiO is also plotted for the comparison. The time dependences are vertically shifted for better clarity. (d) Frequency-domain radiation spectra with the various NiO-mirror distances, normalized by the radiation spectrum of bare NiO. Green arrows denote the radiation enhancement when cavity modes overlap with the magnetic resonance of NiO.

(a) Analytical calculation of on- and off-resonance conditions of the coupled FP cavity. This calculation is performed with the assumption that the permeability of NiO equal to 1. (b) FDTD calculation of the radiation intensity as the function of slab thickness and the NiO-mirror distance, normalized by the intensity of 100 μm thick NiO. (c) Normalized radiation intensity extracted from the best and worst cavity structures at different crystal thickness, forward and backward radiation from bare crystals are also plotted for comparison.